Which will get overwritten on updates to sddm.
Instead of using the distribution file - you should create your own as described in man ssdm.conf
DisplayCommand=
Path of script to execute when starting the display server. The script will be executed as root when General.DisplayServer is “x11”, otherwise as sddm user. Default value is “/usr/share/sddm/scripts/Xsetup”.
The configuration is similar for Wayland
Place a dropin file in /etc/sddm.conf.d with your script
$ cat /etc/sddm.conf.d/00-display-command.conf
[X11]
DisplayCommand=xrandr --output “device” --mode “mode” --rate "rate”
[Wayland]
DisplayCommand=xrandr --output “device” --mode “mode” --rate "rate”
Or you could create an override using /etc/sddm/scripts/Xsetup
sudo mkdir -p /etc/sddm/scripts
Then create the override script
echo xrandr --output “device” --mode “mode” --rate "rate” | sudo tee /etc/sddm/scripts/Xsetup